Weeds

Summary The weeds of farmland are a diverse and economically valuable group of plants that have formed a durable botanical thread since before the land was first opened for cereals and pasture.  Weeds are an economic burden when they take resources from crops, interfere with harvesting or contaminate yield. But only a few plant species cause economic loss at any time, and those few change over the years in response to trends in cropping.  Weeds are not all burden: many species provide food and shelter for insects, some beneficial to agriculture. They mop up fertilisers missed by the crop, so reduce pollution. Leguminous weeds can fix nitrogen from the air, so reduce the need for mineral fertiliser. Many weed species have nutritional or pharmaceutical properties that are still valued in cultures throughout the world. Fifty years of chemical weedkillers have not exterminated weeds in north west Europe. The beneficials have been reduced, but the aggressive few have simply adapted. The future lies in limiting the economic burden of weeds, encouraging their beneficial aspects and discovering new economic products in their biochemistry and structure.

The weeds panel shows: left top to bottom: seeding spikes of rose bay willow herb, flower stalk of sticky mouse-ear (negative x 10), seed pods of wild radish collected from rapeseed harvest, seed of the proscribed weed spear thistle; and right, flower and buds of scented mayweed, fly on chicory flower, assorted seeds, and flower variants of pansy. All photographs from the Living Field collection.

Please go to Sources and contacts for the article 'Weeds and the law' by C and A Reid.

Introduction, global view

Of all the organic things that interfere with growing crops, the in-field weeds are probably the most important worldwide, more important economically than pathogenic diseases and the insects. Because many of the weeds are what are called generalists, they readily invade and adapt to disturbed cropped habitats anywhere. They are weeds because they do one or more of three things: deny the crop resources of solar energy, water and nutrients; interfere with field operations by dragging down the crops or jamming up machinery; contaminate the harvest. Unlike many pathogens and insects that are specialists, weeds regard fields, whether in cereals, legumes, oilseeds or vegetables, as just another opportunity. Some weed species occur throughout vast tracts of land. Others may be a rare wild plant in one region but spead aggressively in another. 

The agricultural weeds come from many different plant families. They tend to release great quantities of pollen, either directly to the air - as in the photograph right of a grass spike emitting pollen - or on the bodies of insects. They reproduce variously by seed and by vegetative materials. The most successful are highly variable genetically, occurring in different forms that - in an evolutionary sense - continually outwit their adversaries. 

However, it’s not just the damaging weeds that cohabit with crops in fields. Many other plant species have taken up residence there that are a negligible threat to crops and more importantly are a benefit both the working of the agro-ecosystem and to human existence through their value as a source of medicines, oils and other substances (for example the mullein shown below). Many a market in eastern europe will have a good selection for sale of what we call weeds. Chinese medicine has taken the use of herbs, some of them weeds, to very high levels. The dye plants such as indigo, woad and weld could all be classed as weeds in some circumstances.

Modern intense agriculture barely distinguishes the economic threats from the rest, and it’s the neutral or beneficial ones that have suffered the greatest declines in the last 50 years. Some are now more common in gardens than in fields. Yet many of them are still around in fields, as we shall see, and have still to reveal their economic potential.

Example of a multi-functional weed - the mullein Verbascum thapsus - showing plant, flower spike and flowers of  this agricultural weed, garden plant, medicinal and food source for insects. It can grow taller than 2 m (7 feet) when left undisturbed, though is now uncommon in agricultural land (Living Field collection).

In Britain, since the ice age

Weeds in Britain and much of northern Europe were here well before agriculture began 6000-5000 years ago. Some followed the retreating ice northwards at the end of the last glaciation. They perhaps reached their lowest ebb when the land was subsequently covered in trees, when they would have retreated to unwooded margins of hills and coasts. Some would have been valued by the first human migrants. The hunters ate weed seeds or knew their medicinal value. A few thousand years later, humans created habitat in the form of clearings and cropped fields for the re-expansion of weeds. They have since been a part of this habitat, adapting to the plough and all means to remove them. The weeds have been here longer than people and longer than crops.

The first resident weeds were soon be joined by continental plants that were separated from the local flora when Britain was cut off from what is now mainland Europe. The first farmers, in the neolithic or stone age, brought weeds along with their cereal seed. Weeds still common such as fat hen Chenopodium album were found deep in stone age monuments such as Newgrange in Ireland, and the poisonous Agrostemma githago was recently unearthed from iron age and later grain storage pits. Each phase of migration and import, even up to the present day, brought with it new species.  By the early nineteen-hundreds, the weed flora had become fairly rich – 50 to 100 species in some fields. Contrast this with the few weed species that occupy newly opened land in north America, for example.

The weeds have been reduced within cropped fields in the last 60 years by more vigorous crops, less open land in fallow and much more targeted chemical control, but they have not been removed from the rural scene. They have survived in crops such as oilseed rape and set-aside, where many weeds are able to complete their life cycle. People have unwittingly created new habitat for them in industrial and urban waste land and along transport corridors.  

The seedbank

A characteristic of weeds in north west Europe is the seedbank of buried seed that they form in agricultural soil. Each species' seedbank persists in the soil for several years, so that even when they appear to have been eradicated, they  come back. While some seeds are repeatedly brought into the seedbank by stock animals and machines, and others are carried on the wind, most of the seedbank is replenished by germination and seeding of what's already there. Because of this, fields rarely have the same weed species in their seedbank. The commonest might be the same but the rarer ones tend to be different, possibly due to past events that have allowed certain types to multiply. 

The seeds take many different forms, as the photograph above right shows. Each seed is a small packet of food, high in energy and nutrients, that nourishes small mammals, birds, insects, fungi, bacteria, nematodes and other soil organisms. Each year these creatures take about half the seedbank. Yes half! The seedbank suffers a high rate of decline. You would think that if farming was very efficient at controlling weeds, if it stopped weeds emerging from the seedbank and producing new seed, the seedbank would disappear. And it would ... but weed control is never absolute - at least some species make it through in most years and in some years many species complete their life cycle often unseen under the crop canopy. 

The photographs above show seeds (not to the same scale) from the reference archive. This collection of photographs and real seed samples displays the great diversity of structure, colour, texture and shape among seeds of the common wild arable plants. The archive is used for identification and training (Living Field collection and ASIS: for further information, see 'sources' below)

When studying the economic and ecological effects of  weeds, some research groups (including ours) use plant functional classifications. This form of classification is based on the life cycle biology and structure of the plants and how they are perceived by crop plants and animals of the food web. For example, life history categories include annual, biennial, perennial; determinate or indeterminate reproduction; upright or spreading habit; and season of germination. In relation to the food web, the plants can be classed as food for leaf chewers, stem borers, pollen and nectar feeders or seed eaters. Sometimes functional classifications group together species of a similar type. At other times, functional classifications split species because they are so variable in their structure that they can't be contained in one functional type. For example, the species shepherd's purse Capsella bursa-pastoris and the knotweed Polygonum aviculare should not be each classed as one unit of biodiversity (that is, 'species') since they consist of many distinct functional types. 

Ecological and economic roles of weeds

The economic burden

In any place, at any time, usually five or six weed species, rarely more than ten, make up the true economic burden, in that, if not controlled, they reduce yield, clog combines or contaminate the harvest. In much of Britain now, the economic weeds cohabit with cereal crops and are mostly grass (also called monocot) species.  Grass weeds are the main problem in cereal (i.e. grass or monocot) crops because the weeds adapt to the life cycle of the crop plant and are hard to kill among other monocots with selective herbicides. Except for the Poa species, and blackgrass in the south, most of these major weeds are not common in the seedbank, presumably because so much effort is directed at getting rid of them. 

  •  wild oat Avena fatua, a common and important annual weed of cereals, the winter wild oat Avena sterilis having a more southerly distribution.
  • the bromes, the annual barren brome Anisantha sterilis (once classified as a Bromus) and soft-brome Bromus hordeaceus, not such important weeds in the north as they are in the south.
  • black grass Alopecurus myosuroides, a major weed in cereals in the south and moving north into Scotland.
  • the common couch Elytrigia repens, a creeping rhizomatous perennial that should be readily controlled by cultivation and herbicide.
  • annual meadow grass Poa annua (now the most common and abundant species in arable fields) and rough meadow grass Poa trivialis a stoloniferous perennial,  both widespread.

Many other grass species are common in farmland whether in pasture or in field margins but are not important weeds in arable fields.

In addition to the grasses, several broadleaf (or dicot) species also cause major economic problems.

  • cleavers or sticky willie Galium aparine, often the most important weed, clinging to crop plants, scrambling over them, dragging them down; the photograph (right, as a negative image) of young plant about 10 cm tall shows its potential - two large seedleaves or cotyledons at the base, a whorl of four leaves above and another of six leaves above that, branches already appearing in the axils of the seed leaves and one bracnch in the first whorl, almost limitless potential within a season to branch and spread.
  • chickweed Stellaria media, can form dense mats that choke the crop.
  • common field speedwell Veronica persica, among many weedy speedwells the only one to seriously challenge a crop.
  • corn spurrey Spergula arvensis, once a fodder crop and human food, and now still an abundant weed in northern low input fields (despite being classed as rare or even at risk in some registers!)

Numerous other broadleaf weeds - another 100 species can readily be found - have adapted to life in arable fields but very few now seriously challenge a strong crop and competent weed management.  Their ecological effects are considered under beneficial weeds.

Infamous weeds of the weeds act 

Certain weeds were such a problem in the arable-grass farming of the first half of the 1900s that the 1959 Weeds Act came into force to enable the authorities to consider taking action against landowners who allowed any of five weed species to flourish. Even today, the authorities are obliged to consider acting if an infestation of these species is reported to them!  See also web pages on Enforcing the weeds act 1959 and for, Scotland Weeds Guidance (2009). The weeds act weeds are -

  • common ragwort Senecio jacobaea, biennial, flowering before dying, usually in the second year; advancing in many places and poisonous to some stock.
  • broad leaf dock Rumex obtusifolius, the commonest of the five in arable fields, perennial, but not a serious arable weed today.
  • curled dock Rumex crispus, perennial, and much as for the broad leaf dock.
  • spear thistle Circium vulgare, perennial, tending to form big single plants or clumps, but usually controlled after the first year in arable fields (photographs below, right).
  • creeping thistle Circium arvense, perennial, now common in gardens, industrial land, and waysides but can still be seen to 'creep' over time, a few metres a year,  through unkempt pasture. 

The weeds act weeds did their economic worst when most of agriculture was either pasture or a mix of arable crops (e.g. cereals) and grass, when in either case the weeds found opportunity to spread. That all five are biennial or perennial, and do not flower until the second year, means they are exposed to modern weed control, which can stop them in the period between one arable crop and the next, if not within a crop's season. Consequently, they are not now a serious weed of arable fields, but are still commonly found there as first-year rosettes. In pastures, especially rough grazing, they are still a nuisance but perhaps more to people who keep horses - the ragwort is poisonous - than to farmers. (Note the link above to the Defra web site is to the wildlife and pets section.) They can still be seen to creep over unkempt pasture, eventually reducing its value. Major roads and motorways now provide almost unlimited corridors for their movement.

In biological terms, the weeds act weeds have been successful. Around in this area for thousands of years before and after the last ice age, they took good advantage of man's intrusion to spread and dominate opened land. They all produce a lot of seed, that of the ragwort and thistles being carried in the air, and all have various defences against being eaten. Apart from the ragwort, mature plants can last for years, decades, even longer. They've been set back in cereal fields by the plough and chemical weedkillers, but have not been driven out. They are part of the rural scene. All are good botanical citizens, supporting the local food web - it's rare to see one in summer without an insect feeding on it. If plants had feelings, the weeds act weeds would be grateful to humans. They'll be here for some time yet.

Poisonous weeds

Plants that contain chemicals poisonous to humans have long cohabited with crops. They were still causing problems until fairly recently, and are still common in fields in parts of eastern Europe. Common sense decreed they had to go from fields, then best practice and modern agricultural science found the means to remove them. Most of them are rarely found in arable-grass seedbanks. They are nevertheless an interesting and useful group, many of which have medicinal properties if prepared and administered in the correct way.

  • corncockle Agrostemma githago, a poisonous contaminant of grain crops, presumed extinct in the UK as a result of rigorous seed purification methods, but reintroduced to wildflower gardens.
  • henbane Hyoscyamus niger, highly poisonous member of the potato family, now uncommon here, but still a weed in parts of eastern europe.
  • hemlock Conium maculatum, rare in fields here but still present in corners, bits of waste land and road verges.  
  •  thorn apple Datura stramonium, occasionally found in fields and waste land probably as an imported impurity, but turns up in the most unexpected places!
  • black nightshade Solanum nigrum, mostly rare, but uncharacteristically for rare weeds, where it occurs it does so in large numbers.

While the poisonous weeds have been depleted, many other poisonous plants remain, and even the fact they are poisonous has been lost from local knowledge.

Beneficial weeds

Even the most intense crop management does not kill all weeds all the time. A few individuals will complete their life cycle and return new seed. So how many species are there. A typical arable field has 10 to 40 seedbank species, of which only a few are of real economic concern. The total number of species might reach 70 or more in a low-input field. Many of the same species recur in different fields, but in total an area such as maritime east Scotland (Caithness to the Borders) supports 200 to 300 species within arable fields and the commoner ones have many functional variants. In addition to this in-field flora are the mostly perennial plants that live in field margins and hedges.

In total therefore, the arable flora is still quite rich and abundant, though steeply declining. The decline is of no economic benefit. The rarer species remain in areas of low input cropping, so in Scotland it is still possible to find large populations of (e.g.) the rare ancient weed, cornflower Centaurea cyanus and the rare introduced weed fiddleneck Amsinckia micrantha. The panel below shows flower structure of weeds of the cabbage family (Cruciferae).

Photographs Floral structures of 5 of the many species of the crucifer (cabbage) family that grow in and around fields: top left, the white flowered wild radish Raphanus raphanistrum; bottom left, charlock Sinapis arvensis; top mid, the flixweed Descurainia sophia, bottom mid, treacle mustard Erysimum cheiranthoides; and right, white mustard Sinapis alba probably derived from a crop. All support the food web, only charlock is an economic weed.

What the beneficial weeds do or could do - 

  • support the food web - per gram or ounce of plant matter, the weeds host at least ten times as many insect species as the crops; and these species include the pollinators, and the predators and parasitoids that feed on the insect pests of crops (in time we'll put a link here to an article on the food web);
  • fix nitrogen (N) from the air into plant compounds so reduce the need for mineral nitrogen fertiliser - Ok, so modern arable farming has driven out the leguminous weeds that fix atmospheric N, but their potential is unquantified;
  • mop up, and return to the soil in organic form, stray fertiliser that the crops do not take up and would otherwise be lost from the field and become pollution;
  • offer uncharted potential as medicinals, pharmaceuticals, oils and fibres.

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Status and future of weeds in Britain

[Under construction]

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Photographs of dandelion showing, left to right, flower bud, emerging flower, seed head full of seed and the same when most seeds have blown away.

Sources and contacts

Books and articles

There are few recent books on weeds. For identification of species, try to get hold of Chancellor or Hanf, both still available second-hand, or else try the web sites below, including our own ASIS site. For readers with a deeper interest, Brenchley's title was the first substantial book on weeds in Britain and gives an idea of the practical and economic problems due to weeds before the pesticide years. The books by Long, also well out of print, give the picture one or two decades after Brenchley. CTW is still the best for summaries of functional form and life cycle biology of most weed species likely to be met.  Those readers interested in more specialist books and papers are welcome to contact the authors.

  • Brenchley WE. 1920. Weeds of farm land. Longman, Green and Co. London. 239 pages.
  • Chancellor RJ. 1966. The identification of weed seedlings of farm and garden. Blackwell Oxford. 88 pages. (One of the best for identification of seedlings.)
  • Clapham AR, Tutin TG, Warburg EF. 1952. 2nd ed. 1962. Flora of the British Isles. Cambridge University Press. 2nd edn 1269 pages. (Later revisions with changed authorship are available.)
  • Grigson G. 1958. The Englishman's flora. Paperback published 1975 by Paladin, St Albans, UK. 542 pages.
  • Hanf M. 1983. The arable weeds of Europe with their seedlings and seeds 1983 BASF. [http://web.adas.co.uk/WeedManager/FurtherReading.aspx].
  • Hubbard CE. 1954. Grasses. Second edition 1968. Penguin Books. 463 pages. (There are later, revised editions with different authors.)
  • Hutchinson J. 1955. British wild flowers. Volumes I and II. (Pelican) Penguin Books. 947 pages.
  • Long HC. 1929. Weeds of arable land. HMSO London. By the same author: 1927. Poisonous plants on the Farm. HMSO London; 1938. Weeds of grass land. HMSO London.  (Useful still, but hard to find. Good black and white drawings and photographs, some in colour in the one on grass land.)
  • Ranson F. 1949. British herbs. (Pelican) Penguin Books. 203 pages.
  • Stace C. 1997 New flora of the British Isles, second edition. Cambridge University Press. 1130 pages. ISBN 0 52158933 5, plastic covers. (Current standard reference for plant names and taxonomic status.)

Botanical Society of the British Isles (BSBI) guides The BSBI have a set of invaluable handbooks on comon plant families or groups, including -

  • Lousley JE, Kent, DH. 1981. Docks and knotweeds of the British Isles. BSBI London. 205 pages. Paperback ISBN 0 901158 04 6.
  • Rich TCG. 1991. Crucifers of Great Britain and Ireland. BSBI London. 336 pages. Paperback ISBN 0 901158 20 8.
  • Tutin TG. 1980. Umbellifers of the British Isles.  BSBI London. 197 pages. Paperback ISBN 0 901158 02 X.

Weeds and the Law

Colin Reid of Dundee University has kindly allowed us to make the following article available via a pdf. His book on Conservation Law contains additional material.

Reid CT, Reid MA. 1990. Weeds and the law. Journal of the Royal Caledonian Horticultural Society, (1990) 47-49. Pdf file 300 kb.

Web sites and databases 

The Plants For A Future web site has a searchable database on plants including many of those classed as weeds.

 

At SCRI

The information on this page is based on the Institute’s work on arable seedbanks, initiated by Harry Lawson (now retired), and Gladys Wright, and carried out since the 1980s. Summaries of major seedbank projects and surveys can be viewed on the Institute’s web page on the seedbank. Major databases on seedbanks and weed vegetation are maintained to help research on sustainable food production and biodiversity. An online guide called ASIS was constructed to enable people to identify the common weeds. The Living Field garden provides refuge for and a teaching resource on weed species.

ASIS (Arable Seed Identification System) is a free, web based guide to weed seeds, later extended to include seedlings and mature plants. It was developed by Gladys Wright and Bruce Marshall at SCRI and a group of students at the University of Abertay Dundee. The original online system is still current and will be updated with additional species. See it at ASIS.

Photographs of seeds and plants shown on this web page are from the Living Field's own collection. Many of them, including those with a black background, were taken by the (then) SCRI staff photographers, notably Stuart Malecki, from material prepared by Gladys Wright and helpers. Please respect their work and do not use photographs from this page or any other part of the Living Field site without getting permission. Contact: Gladys Wright.

The Arable Plants Document is a summary of recent government-funded research on the status, value and management of weeds in Britain, to be released in various formats in 2010 and 2011, including this web page on the weeds. Coordinator of the Arable Plants DocumentPietro Iannetta.

Recording of seedbanks and arable vegetation is a specialist skill learned and practiced at SCRI by (alphabetically) Cathy Hawes, Gillian Banks, Gladys Wright, Linda Ford and Geoff Squire. In addition, Pietro (Pete) Iannetta and Alison Karley contribute specialist knowledge of weed biology. 

Acknowledgements

Among those farmers and others who have provided us with more than routine enlightenment on weeds, we are particularly grateful to the following.

Mrs Shirley Harrison, New Craig Farm, Aberdeenshire, for the bags of wild radish Raphanus raphanistrum seed, the amusing and not inconsequential story of how they came to be in her oilseed rape harvest, for hosting assorted field trials mostly of a contentious nature, and for many happy hours discussing the finer points of weed management.

Mr Edward Baxter, Fife, for hosting many events and trials, for being the force behind LEAF Scotland for the last ten years and not least, in patiently listening to, but gently though repeatedly channelling to practical purpose our wilder ambitions with respect to the future of weeds in society.

Mr Iain Cummings, crofter, late of Clachandruim, Bunachton, who showed that profit is not all, and that if you want to limit the weeds act weeds in poor pasture you have to deal with them one by one every year or so, even if it takes a bit of effort.

Contact for this page: Geoff Squire

[Page began 20 October 2010.  Live 2 November 2010. Last updated 22 July 2011.]

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